The overall objective of this project is to elucidate pathways by which antigen presenting cells process exogenous antigens for presentation to T cells. The specific aims are: 1) to characterize the defects in B cell line mutants which are unable to present HLA class II restricted antigens to antigen specific T cells; 2) to clone genes involved in antigen processing; 3) to investigate the "occupancy" hypothesis for the effect of mutations in antigen processing genes on the conformation of HLA class II molecules; and 4) to study combined immunodeficiency patients for defects in antigen processing. The basic strategy for cloning antigen processing genes will be to transfect various cloned genes and gene libraries into the antigen-processing mutants and to select for mutants whose defect has been complemented by the introduced DNA. Immunomicroscopic and biochemical approaches will be used to characterize the defects in antigen processing mutants. The "occupancy" hypothesis will be tested by binding studies of peptides to class II molecules from antigen processing mutants. Cell lines from patients with combined immunodeficiency of unknown etiology will be analyzed for defects in antigen processing. It is anticipated that detailed characterization of the basic defects in these mutants and the cloning of the normal homologues of their mutant genes, as proposed here, will yield important information on antigen processing, on class II biosynthesis and expression, and on the nature of allorecognition.